XRCC6 and cancer: E3 ubiquitin ligases (Mule, SCFβ-TrCP, SCFFBW7, TRIM17, APC/CCdc20, and FBXO4) and deubiquitinases (USP9X, Ku70, USP13, JOSD1, and DUB3) work together to balance MCL1 stability, which has an important role in the chemoresistance of cancer cells [35,36].